This chart is from a Bureau of Labor Statistics working paper that was released in 2008, titled "Self-Employment Transitions among Older American Workers with Career Jobs." I know that's from a few years back — and the data itself is drawn from the 1997-2004 period — but you can see a clear trend: As workers age, they increasingly consider moving into self-employment. Particularly men. Some of this probably has to do with ongoing financial needs in retirement. But the data comes from a study of people aged 51-61, so the "transition" from full-time work to self-employed work is taking place well before the typical retirement age.
Now connect this trend with what Sara Horowitz, founder of the Freelancers Union, has recently been saying about the changing nature of work in the Atlantic. You can then see that the "Gig Economy" isn't just about younger workers who are stuggling to find full-time jobs. It's also about older workers who are seeking new ways to work, either out of desire or necessity.